Multi mains and ploy crosses:
Multifilaments are designed for comfort above all else, with a secondary benefit of having slightly better tension maintenance than syngut. Poly is the compromise between the stiffness of Kevlar and the elasticity of nylon.

Having multifilament mains in a hybrid is counterproductive because you're not gaining the spin-potential of the stiffer poly, and any increase in the overall life of the stringbed is marginal, at best. All you're doing is lowering the power of the stringbed.

Poly mains and multi crosses:
This setup has greater potential with the poly in the place where you would need it. The mains. Multi crosses soften up the feel somewhat, though your stringer is correct in saying the mains influence the feel of the stringbed more. The real benefit of multi crosses here is the nylon material is more flexible, thus allowing the stiff mains to bend out of place and snap back more easily.

Gut mains and poly crosses:
Gut has the best resiliency (elasticity) of any string material. It stretches better and does so longer and more consistently than nylon, Kevlar, poly, piano wire, you name it. It of course is also the most comfortable. Gut is an ideal material for mainstrings when combined with poly...

Unfortunately you are wrong in some of what you say. Multi(filament) type strings are designed initially for performance & playability. They are a 'true' syn gut as they try and mimic natural gut. One of those properties is the comfort of the string I'll give you that, but that is not its main goal. Multis don't have as good as tension maintenance as regular nylon syn guts. With tension maintenance gut is king, followed by syn gut, multis then polys.

While you say that having the multi in the mains is counterproductive, it is not. As previous mentioned, most of the feel of the string bed comes from the mains strings. A multi/poly hybrid is merely an alternative set up to a poly/multi. Each set up gives differing characteristics and players using hybrids need to find which set up suits them better. One may only be better than the other only in respect of the player.

You mentioned that by having multis in the mains you lower the power. So by your words that would imply that a poly/multi would be more powerful. That is incorrect. As the main characteristics of the stringed comes from the mains, and multis are more powerful than polys, having a multi there would be more powerful. A full bed of a multi is more powerful than a full bed of poly.

A benefit of having multis or indeed syn gut in the crosses of a hybrid is to reduce the stiff(er) feel of full poly.

A gut/poly hybrid is a good choice because of the benefits of gut. Since nat gut & multis are similar as I've mentioned before, surely there's no reason to dismiss a multi/poly hybrid as you've done. In fact a multi/poly can be a cheaper alternative to gut/poly hybrid to those trying to get a similar type feel.